Astrophysicists have discovered a galaxy that contains very little dark matter, which funnily enough, might help to prove that dark matter exists.
A team led by researchers from Yale University noticed the galaxy, NGC 1052-DF2, because it looked rather strange in images from the Dragonfly Telephoto Array.
To get a better picture, the team used the WM Keck Observatory in Hawaii, NASA’s Hubble Space Telescope and the Gemini Observatory in Hawaii. What they found utterly defied expectations.
“It’s so rare, particularly these days after so many years of Hubble, that you get an image of something and say, ‘I’ve never seen that before’ . This thing is astonishing, a gigantic blob that you can look through. It’s so sparse that you see all of the galaxies behind it. It is literally a see-through galaxy.”
On top of its strange ghostly appearance, the galaxy also proved to have almost no dark matter, Van Dokkum adds.
We thought that every galaxy had dark matter and that dark matter is how a galaxy begins. This invisible, mysterious substance is the most dominant aspect of any galaxy, so finding a galaxy without it is unexpected. It challenges the standard ideas of how we think galaxies work, and it shows that dark matter is real. It has its own separate existence apart from other components of galaxies. This result also suggests that there may be more than one way to form a galaxy.”
Dark matter cannot currently be “seen” by us, the only thing that proves it exists is its gravitational interactions with normal matter. Nevertheless, it is believed to make up around 80 percent of the mass of the Universe and many theories also suggest that dark matter is integral in the formation of galaxies.
According to computer modeling, galaxies typically form when dark matter merges together. The normal matter that we see around us combines within this net of dark matter in the form of stars and galaxies.
But this galaxy doesn’t look like it has formed that way at all. Motion measurements from the WM Keck Observatory show that the dense, globular clusters of stars in NGC 1052-DF2 are moving very slowly, at less than 23,000 miles per hour. Stars in galaxies with dark matter move at least three times faster than that.
“If there is any dark matter at all, it’s very little,” van Dokkum explains. “The stars in the galaxy can account for all the mass, and there doesn’t seem to be any room for dark matter.”
So how did the galaxy form without dark matter as its scaffolding? And why is this galaxy the same size as our Milky Way, but contains only 1/200th of the number of stars?
The researchers put forward two possible explanations for the unique nature of this galaxy. NGC1052–DF2 was formed in a cluster of galaxies that’s dominated by a giant elliptical galaxy and it’s possible that DF2 was formed by leftover gas from the mergers of some of these galactic neighbours.
It’s also possible that some sort of cataclysmic event within the galaxy, such as the eruption of a star formation, might have cleared out all the gas and dark matter, leaving just the stars behind.
“We thought all galaxies were made up of stars, gas and dark matter mixed together, but with dark matter always dominating,” says Roberto Abraham, a professor in the University of Toronto’s department of astronomy and astrophysics, and co-author of the study.
“Now it seems that at least some galaxies exist with lots of stars and gas and hardly any dark matter. It is pretty bizarre .”